In continuous casting of hypo-peritectic steel whose C concentration is 0.08 to 0.18 mass %, a solidified shell that is formed from solidification of molten steel in a mold tends to be unequal in thickness, which causes longitudinal cracks to easily form on a surface of a slab.
Upon continuous casting, it is effective to mildly cooling an edge portion of a solidified shell (hereinafter referred to as “mild cooling”) in order to make the solidified shell in the mold equal in thickness. It is relatively easy therefor to use mold flux.
Mold flux is supplied to the surface of molten steel that is poured into a mold, melts with heat supplied from the molten steel, and flows along the mold, to infiltrate into a gap between the mold and the solidified shell, and to form a film. Just after casting is started, this film is cooled by the mold, to solidify like glass. Crystals are educed from the glass as time passes. If crystallization of this film is promoted, the roughness of the surface of the film in the mold side increases, which causes the interfacial thermal resistance between the mold and the film to increase. In addition, radiative heat transfer in the film is suppressed. Thus, these effects allow the molten steel and the solidified shell that touch the film to be mildly cooled down.
It is cuspidine (Ca4Si2O7F2) that is common composition of crystals educed from the film.
The following methods have been worked out upon promoting crystallization of the film:
First, if fluid physical properties of mold flux are controlled, it is an effective method for promoting crystallization to raise the solidification temperature. Patent Literature 1 discloses that the crystallinity is improved by raising the solidification temperature to the range of 1150 to 1250° C.
Patent Literature 1 describes that if the solidification temperature of the mold flux is raised more than 1250° C., the lubricity between the mold and the solidified shell is disturbed and breakout (the solidified shell breaks and the molten steel is leaked) cannot be prevented.
When chemical components in mold flux are controlled, it is effective to increase the mass concentration ratio of CaO to SiO2 (hereinafter referred to as “basicity”). It is also effective to reduce the MgO concentration.
For example, Patent Literature 2 describes it is effective for crystallization of a film that the basicity is specified by 1.2 to 1.6 and then the MgO concentration is specified by no more than 1.5 mass %. However, since the highest temperature where crystals form in Examples of the mold flux described in Patent Literature 2 is 1145° C., only an effect of mild cooling corresponding to this is obtained.
On the other hand, Patent Literature 3 discloses a method for suppressing radiative heat transfer in a film by adding an iron oxide and/or a transition metal oxide to mold flux.
However, CaO, SiO2 and CaF2 in the mold flux are diluted by the addition of any of these oxides. Specifically, in the invention of Patent Literature 3, no less than 10 mass % of an iron oxide and/or a transition metal oxide in total has to be added as shown in its Examples in order to obtain a sufficient effect of suppressing radiative heat transfer. In this case, cuspidine is difficult to be educed when the basicity of the composition is about 1.0, which is shown in the Examples, and the solidification temperature of the mold flux drops.
The solidification temperature in Examples described in Patent Literature 3 is about 1050° C., which is lower than that of the mold flux for hypo-peritectic steel by no less than 100° C., considering that the solidification temperature of the mold flux for hypo-peritectic steel is about 1150 to 1250° C. as described above. Therefore, as a result, crystallization of the film is blocked. Thus, an effect of mild cooling according to crystallization, that is, increase of interfacial thermal resistance and the like, is marred.
Patent Literature 4, which was formerly proposed by this inventor, discloses a range of the composition of mold flux where cuspidine is easily educed, in the quaternary system of CaO—SiO2—CaF2—NaF. This range of the composition is substantially same as a primary crystallization field of cuspidine according to the report thereafter (ISIJ International, 42 (2002), p. 489).
This inventor also proposes, in Patent Literature 5, the method for adding a transition metal oxide to the basic composition prepared within the range of the invention of Patent Literature 4, to drop the solidification temperature without marring an effect of mild cooling.
The invention proposed in Patent Literature 5 is to obtain an effect of mild cooling that mold flux whose solidification temperature is 1250° C. or more has, which has been conventionally considered to be difficult to be used because the lubricity is disturbed, from the solidification temperature of the general range, 1209 to 1239° C. as illustrated in its Examples, for example.
However, in casting of hypo-peritectic steel as well, longitudinal cracks more easily form when the degree of superheat (which means “difference between the temperature of the molten steel and liquidus temperature”. Hereinafter the same will be referred to.) of molten steel is high, as the case when a grade of steel containing an alloying element such as Cu, Ni, Cr, Mo, Nb, V, Ti and B.
When the degree of superheat of molten steel is high upon casting hypo-peritectic steel as the above, an effect enough for preventing or suppressing longitudinal cracks might not be obtained even with the inventions of Patent Literatures 4 and 5 proposed by this inventor. That is, a sufficient effect might not be obtained even from mold flux where cuspidine is easy to be educed by adding a transition metal oxide to its composition range, when hypo-peritectic steel containing an alloying element such as Cu, Ni, Cr, Mo, Nb, V, Ti and B is continuous-cast.